FIG. 1 shows a simplified plan view of a conventional package housing a power MOSFET die. FIG. 1A shows a simplified cross-sectional view of the package of FIG. 1, taken along line 1A-1A′.
Specifically, the conventional power MOSFET package 100 comprises power MOSFET die 102 having a top surface and featuring gate pad 104 and source pad 106. Gate pad 104 is configured to be in electrical communication with first lead 110 through bond wire 112, and source pad 106 is configured to be in electrical communication with second lead 114 through bond wire 116.
The bottom surface of die 102 features the drain pad 108. The drain pad is in electrical communication with an underlying die pad 118 through an electrically conducting adhesive material 120. This adhesive material 120 is also thermally conductive, allowing heat generated by the MOSFET die during operation, to be transported out of the package through the heat sink 122 formed by the lower surface of the die pad. Thermal energy may also be conducted out of the package through the leads that are integral with the die pad.
While the package of FIGS. 1-1A is functional, it may offer certain drawbacks. One drawback is the requirement to perform wire bonding between the pads or the surface of the die, and the leads. Specifically, this wire bonding step is costly, as the bond wire material is typically made from gold, a highly expensive commodity.
The wire bonding step is also difficult to perform, as it requires the bond wires to be bent (strained) and then attached with some force and with high precision to the die and to a small target area at the ends of the leads. Fracture of the wire under the strain, or failure to accurately align the wire end, can enhance defects and reduce throughput. The force of attachment of the wire to the die in this step can also harm the die.
Moreover, the source and gate bond wire connections limit the ability of the package to dissipate thermal energy. In particular, the small volume of the bond wires offers only a small volume of thermally conducting material to transport heat out of the package.
Finally, the relatively small cross-sections offered by the bond wires may interfere with establishing a low resistance contact between the die and the leads. Conventional efforts to establish lower resistance contacts often amount to the use of more bond wires, exacerbating the cost issues described above. Moreover, use of multiple stitches of a bond wire to establish a low resistance contact the die surface, requires multiple attaching steps that again pose the danger of possibly damaging the die.
Other disadvantages may be offered by the use of long and/or multiple bond wires as electrical connection to the die. For example bond wires may offer larger inductance that can impair the switching behavior of MOSFETs. Also, bond wires can add uncontrolled external inductance or impedance to a Power IC, which would require compensation in the internal integrated circuitry.
Accordingly, there is a need in the art for improved package designs exhibiting favorable heat conduction and low cost fabrication.